The present invention relates to an electrophotographic developer and more particularly to a two-component electrophotographic developer containing a toner and a carrier, to be used for an image forming apparatus such as an electrostatic copying apparatus, a laser beam printer or the like.
In the image forming apparatus above-mentioned, the surface of a photoreceptor is exposed to light to form an electrostatic latent image on the surface of the photoreceptor. By a developing device, an electrophotographic developer is let come in contact with the surface of the photoreceptor The toner contained in the electrophotographic developer is electrostatically sticked to the electrostatic latent image, so that the electrostatic latent image is formed into a toner image. From the photoreceptor surface, the toner image is transferred to and fixed on paper. Thus, an image corresponding to the electrostatic latent image is formed on the paper surface.
As the electrophotographic developer, there is generally used a two-component developer containing a toner and a carrier which is adapted to circulate in the developing device while adsorbing the toner.
As the toner, there may be used one as obtained by blending a fixing resin with a coloring agent such as carbon black, a charge controlling agent and the like and by pulverizing the blended body into particles having sizes in a predetermined range.
As the carrier, there may be preferably used a carrier having a core material made of iron particles or the like, of which surface is coated with a coating resin. The object of such coating of the carrier core material at the surface thereof with a coating resin is to control the toner electric charge amount and polarity, improve the dependency of the developer electric charge on humidity and prevent the occurrence of filming.
As the fixing resin and the coating resin, a styrene-acrylic copolymer may be suitably used in view of ease of handling and the like.
A conventional electrophotographic developer presents the following problems That is, when the developing operation is repeated, the developer is subjected to a mechanical pressure, an impact force, friction and the like in the developing device, causing the developer to be gradually deteriorated. This provokes the problems that the electric charging characteristics become unstable, the resultant image is deteriorated in quality and the toner consumption is increased.
As above-mentioned, when the developer is subjected to a mechanical pressure, an impact force, friction and the like in a developing device, the toner particles are crushed and the carrier coating resin falls or partially comes off from the carrier core material, thereby to produce defective fine particles. Such defective particles deteriorate the image in quality. More specifically, the defective particles agglomerate with the toner to form toner agglomerates having great particle sizes. Such toner agglomerates make the resultant image coarse to deteriorate the image quality. Further, the toner agglomerates as repeatedly agitated, are gradually grown to giant particles. When the toner image is transferred to paper, such giant particles are caught between the photoreceptor and the paper to form gaps therearound. This provokes a so-called blanking phenomenon that white portions are left on the image without the toner transferred to the paper. If a great amount of defective particles or toner agglomerates is formed, the toner consumption is accordingly increased and the toner density becomes unstable.
As mentioned earlier, the electric charging characteristics become unstable because the carrier coating resin falls down or partially comes off to injure the smoothness of the carrier surface, so that the carrier surface conditions undergo a change. More specifically, when the carrier surface is decreased in smoothness, this provokes a so-called spent toner that the toner as sticked to the carrier surface cannot come off therefrom. Further, the defective particles as interposed between the carrier and the toner accelerate the adhesion therebetween, provoking the increase in the amount of the spent toner. Such increase makes the developer electric charge uneven so that the developer is liable to be gradually lowered in electric charge. As a result, the electric charging characteristics become unstable. This provokes the problems that the image density becomes unstable and the toner is scattered to produce fog.